Acid−Base Reactions of Methylnickel Hydroxo, Alkoxo, and Amide Complexes with Carbon Acids. Studies on the Reactivity of Noncyclic Nickel Enolates

Skeletal metalation of lactams through a carbonyl-to-nickel-exchange logic

Classical metalation reactions such as the metal-halogen exchange have had a transformative impact on organic synthesis owing to their broad applicability in building carbon-carbon bonds from carbon-halogen bonds. Extending the metal-halogen exchange logic to a metal-carbon exchange would enable the direct modification of carbon frameworks with new implications in retrosynthetic analysis. However, such a transformation requires the selective cleavage of highly inert chemical bonds and formation of stable intermediates amenable to further synthetic elaborations, hence its development has remained considerably challenging. Here we introduce a skeletal metalation strategy that allows lactams, a prevalent motif in bioactive molecules, to be readily converted into well-defined, synthetically useful organonickel reagents. The reaction features a selective activation of unstrained amide C-N bonds mediated by an easily prepared Ni(0) reagent, followed by CO deinsertion and dissociation under mild room temperature conditions in a formal carbonyl-to-nickel-exchange process. The underlying principles of this unique reactivity are rationalized by organometallic and computational studies. The skeletal metalation is further applied to a direct CO excision reaction and a carbon isotope exchange reaction of lactams, underscoring the broad potential of metal-carbon exchange logic in organic synthesis.

Site-Selective C-C Cleavage of Benzocyclobutenones Enabled by a Blocking Strategy Using Nickel Catalysis

Controlling the chemo- and regioselectivity of transition-metal-catalyzed C-C activation remains a great challenge. The transformations of benzocyclobutenones (BCBs) usually involve the cleavage of C1-C2 bond. In this work, an unprecedented highly selective cleavage of C1-C8 bond with the insertion of alkynes is achieved by using blocking strategy via Ni catalysis, providing an efficient method for synthesis of 1,8-disubstituted naphthalenes. Notably, the blocking group could be readily removed after the transformation.

From acetone metalation to the catalytic α-arylation of acyclic ketones with NHC-nickel(II) complexes

Air-stable N-heterocyclic carbene-nickel(ii) complexes at concentrations as low as 1 mol% exhibit high catalytic activity for the α-arylation of acyclic ketones and join a highly restricted list of nickel catalysts for this key reaction. Mechanistic investigations suggest a radical pathway.

Synthesis, electronic structure, and reactivity of strained nickel-, palladium-, and platinum-bridged [1]ferrocenophanes

The group 10 bis(phosphine)metalla[1]ferrocenophanes, [{Fe(η(5)-C(5)H(4))(2)}M(Pn-Bu(3))(2)] [M = Ni (4a), Pd (4b), and Pt (4c)], have been prepared by the reaction of Li(2)[Fe(η(5)-C(5)H(4))(2)]·tmeda (5, tmeda = N,N,N',N'-tetramethylethylenediamine) with trans-[MCl(2)(Pn-Bu(3))(2)] [M = Ni (trans-6a) and Pd (trans-6b)] and cis-[PtCl(2)(Pn-Bu(3))(2)] (cis-6c), respectively. Single crystal X-ray diffraction revealed highly tilted, strained structures as characterized by α angles of 28.4° (4a), 24.5° (4b), and 25.2° (4c) and a distorted square planar environment for the group 10 metal center. UV/visible spectroscopy and cyclic voltammetry indicated that all three compounds had smaller HOMO-LUMO gaps and were more electron-rich in nature than ferrocene and other comparable [1]ferrocenophanes. DFT calculations suggested that these differences were principally due to the electron-releasing nature of the M(Pn-Bu(3))(2) metal-ligand fragments. Attempts to induce thermal or anionic ring-opening polymerization of 4a-c were unsuccessful and were complicated by, for example, competing ligand dissociation processes or unfavorable chain propagation. In contrast, these species all reacted rapidly with acids effecting clean extrusion of the bis(phosphine)metal fragment. Carbon monoxide inserted cleanly into one of the palladium-carbon bonds of 4b to afford the ring-expanded, acylated product [{Fe(η(5)-C(5)H(4))(η(5)-C(5)H(4))(CO)}Pd(Pn-Bu(3))(2)] (10). The nickel analogue 4a, however, afforded [Ni(CO)(2)(Pn-Bu(3))(2)] whereas the platinum-bridged complex 4c was inert. Remarkably, all compounds 4a-c were readily oxidized by elemental sulfur to afford the [5,5']bicyclopentadienylidene (pentafulvalene) complexes [{η(4):η(0)-C(5)H(4)(C(5)H(4))}M(Pn-Bu(3))(2)] [M = Ni (11a)] and [(η(2)-C(10)H(8))M(Pn-Bu(3))(2)] [M = Pd (11b) and Pt (11c)] by a formal 4-electron oxidation of the carbocyclic ligands. Compounds 11b and 11c represent the first examples of [5,5']bicyclopentadienylidene as a neutral η(2)-ligand. The relative energies of η(2)-coordination with respect to that of η(4):η(0) bonding were investigated for 11a-c by DFT calculations.